Technical Field
The present invention relates to an online cleaning system used to clean a tube and shell heat exchanger including a cleaning system comprising a positioner, a plunger, an umbilical cleaner, and a motor. The cleaning system uses a tube that contains both rotating and translating mechanical actions and cleans while the heat exchanger is in operation.
Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
There are several types of heat exchangers used in various industries. A common type is known as a shell and tube type. Modern shell and tube exchangers are of several types, including: (1) a straight through version where the heat exchange tubes are generally straight, (2) a U-tube version where the heat exchange tubes are bent into a U so the inlets and outlets of the heat exchange tubes pass through the same tube sheet and open into compartments provided by a channel and (3) a floating head type where the inlets and outlets are at one end of the exchanger, the tubes are straight and open, at the opposite end of the exchanger, into a floating head or manifold that directs flow back toward the outlet. U-tube type heat exchangers have a cost advantage because only one set of inlet/outlet channels is required. Straight through heat exchangers are typically selected when the tube side fluid deposits materials in the tube or is corrosive because it is usually more difficult to clean the curve in a U-tube type.
Fixed-tube-sheet exchangers are used more often than any other type. The tube sheets are welded to the shell. Usually these extend beyond the shell and serve as flanges to which the tube-side headers are bolted. This construction requires that the shell and tube-sheet
There is no limitation on the number of tube-side passes. Shellside passes can be one or more, although shells with more than two shell-side passes are rarely used.
Tubes can completely fill the heat-exchanger shell. Clearance between the outermost tubes and the shell is only the minimum necessary for fabrication. Between the inside of the shell and the baffles some clearance must be provided so that baffles can slide into the shell. Fabrication tolerances then require some additional clearance between the outside of the baffles and the outermost tubes. The edge distance between the outer tube limit (O.T.L.) and the baffle diameter must be sufficient to prevent vibration of the tubes from breaking through the baffle holes. The outermost tube must be contained within the O.T.L. Another type of shell and tube heat exchanger is a U-tube heat exchanger. In a U-tube heat exchanger, the tube bundle consists of a stationary tube sheet, U-tubes (or hairpin tubes), baffles or support plates, and appropriate tie rods and spacers. The tube bundle can be removed from the heat-exchanger shell. A tube-side header (stationary head) and a shell with integral shell cover, which is welded to the shell, are provided. Each tube is free to expand or contract without any limitation being placed upon it by the other tubes.
The U-tube bundle has the advantage of providing the minimum clearance between the outer tube limit and the inside of the shell for any of the removable-tube-bundle constructions. Clearances are of the same magnitude as for fixed-tube-sheet heat exchangers.
The number of tube holes in a given shell is less than that for a fixed-tube-sheet exchanger because of the limitations on bending tubes of a very short radius.
The performance of shell and tube heat exchangers degrades over time by the deposition of solids from the tube side flow onto the inside wall of the heat exchanger tubes. This is commonly referred to as tube side fouling and can significantly impair the performance of heat exchangers. Fouling deposits act as an insulator and thereby reduce heat transfer across the walls of the tubes. This fouling can also cause increased pressure drops across the tubes thereby decreasing flow through the tubes. Under certain conditions, these deposits can also promote corrosion of the inside of the tube wall, a phenomenon known as under-deposit corrosion. This corrosion, if left unchecked, can produce leak paths through the tube wall allowing commingling of the heat exchange fluid and the process fluid. Even though tube side fouling is a persistent maintenance problem, it is much preferred to shell side fouling because it is much easier to clean and inspect the interior of the heat exchange tubes as compared to the outside. For this reason, in situations where one of the two fluids is more corrosive or more prone to produce deposits in the heat exchanger, this fluid may preferably be put through the tubes rather than through the shell.
Over time, heat exchangers tend to develop residue on the surfaces of the tubes, tube sheets, tube support plates and other internal structural parts. The residue can comprise adherent films, scales, sludge deposits, corrosion and/or other similar materials. Over time, this residue can have an adverse affect on the operational performance of the exchangers. The same problem can arise for all piping and tubing found in industrial facilities.
Various methods have been developed to clean the inside of heat exchanger tubes to remove deposits. These deposits are often relatively hard and therefore difficult to remove from the tube walls. To effectively clean tube side fouling, the heat exchanger is usually taken off-line and out of service to access and mechanically clean the inside of the tubes. These off-line methods of cleaning include high pressure water cleaning known as hydroblasting, mechanical cleaning using brushes, scrapers or projectiles, and blasting with abrasive media. Once the tubes are cleaned and while the heat exchanger is off-line, the tubes may be inspected to determine if corrosion has thinned or pitted the tube wall and a determination can be made to replace or retain the tube. In some circumstances, the tube may be replaced or simply plugged, i.e. a plug is placed in the tube to block flow through it.
Most inspection techniques require the heat exchanger to be out of service. Cleaning by circulation of abrasive media may conventionally be done while a heat exchanger is in operation by inserting media into the flow entering the tubes and then separating the media from flow out of the tubes. As currently practiced, heat exchangers must be out of service in order to plug a leaking or unserviceable tube. The cost of disassembling and then reassembling the heat exchanger to permit access to the tubes for cleaning and inspection can be significant. More significant in many situations is the lost production cost from taking the heat exchanger and its associated equipment out of service.
Other manual methods involve taking the heat exchanger off-line and out of service to manually clean the tubes. These manual methods of cleaning include: high pressure water cleaning to blast away the deposits, acid cleaning to loosen or dissolve the deposits, or the propulsion of a brush or scraping implement through the tube to scrape off the deposits.
Another common method involves the controlled application of high pressure water and/or chemical streams to the affected areas of the heat exchanger. This method can require the presence of one or more persons at or near the point of application of the high pressure stream to the exchanger during the cleaning process.
For example, an operator may stand in clear view of, and near the line-of-fire of, the high pressure stream to direct the stream to the affected areas of the exchanger. Another person may be needed to operate a control panel next to the exchanger to further control the direction and volume of stream flow. This type of work is extremely labor intensive and potentially hazardous. For example, it may be necessary for crews to manually reposition the device providing the high pressure stream for each cleaning stroke. Further, those persons in close proximity to the cleaning environment can be exposed to high pressure water, hazardous cleaning chemicals or other potentially toxic, poisonous or volatile materials.
All of these manual methods result in the loss of use of the heat exchanger during cleaning and incur the cost associated with the cleaning itself. Furthermore, after cleaning and during operation, the tubes begin to foul and continue fouling resulting in a reduction in heat transfer until the next cleaning. In the case of acid cleaning, pitting and corrosion of the tube may occur.
The costs associated with reduced capacity of heat exchanger tubes can also be substantial in situations where the throughput of process fluids has to be curtailed. In one oil refinery, the estimated lost production costs of reduced throughput from a catalytic cracker due to deteriorating heat exchange performance has been in the range of $500,000/year.
Other methods for cleaning the tubes without taking the heat exchanger out of service include devices which introduce a number of tube cleaners (e.g., balls or brushes) into the fluid which passes through the tubes. The tube cleaners are designed to fit tightly enough into the tube to contact the tube wall while still being pushed through the tube by the fluid pressure. At the outlet of the tube these tube cleaners are collected and recycled back to the tube inlet. In some systems the tube cleaners are propelled through the tube in a direction opposite the fluid flow by reversing the fluid flow temporarily. The number of tube cleaners used and the recycle rate may vary depending upon the cleaning effectiveness desired.
While these on-line systems avoid having to take the heat exchanger out of service, there is significant cost associated with the necessary piping and valving. Further, these methods are prone to plugging of the tube by debris that has been loosened by the tube cleaners. After the tubes have been cleaned the pressure drop across the tube and the heat transfer rate across the tube wall return to their nominal design points.